<s>
In	O
computer	B-Architecture
networking	I-Architecture
,	O
the	O
Datagram	B-Protocol
Congestion	I-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
DCCP	B-Protocol
)	O
is	O
a	O
message-oriented	O
transport	B-Architecture
layer	I-Architecture
protocol	O
.	O
</s>
<s>
DCCP	B-Protocol
implements	O
reliable	O
connection	O
setup	O
,	O
teardown	O
,	O
Explicit	O
Congestion	O
Notification	O
(	O
ECN	O
)	O
,	O
congestion	O
control	O
,	O
and	O
feature	O
negotiation	O
.	O
</s>
<s>
The	O
IETF	O
published	O
DCCP	B-Protocol
as	O
,	O
a	O
proposed	O
standard	O
,	O
in	O
March	O
2006	O
.	O
provides	O
an	O
introduction	O
.	O
</s>
<s>
DCCP	B-Protocol
provides	O
a	O
way	O
to	O
gain	O
access	O
to	O
congestion-control	O
mechanisms	O
without	O
having	O
to	O
implement	O
them	O
at	O
the	O
application	B-Application
layer	I-Application
.	O
</s>
<s>
It	O
allows	O
for	O
flow-based	O
semantics	O
like	O
in	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	B-Protocol
)	O
,	O
but	O
does	O
not	O
provide	O
reliable	O
in-order	O
delivery	O
.	O
</s>
<s>
Sequenced	O
delivery	O
within	O
multiple	O
streams	O
as	O
in	O
the	O
Stream	B-Protocol
Control	I-Protocol
Transmission	I-Protocol
Protocol	I-Protocol
(	O
SCTP	B-Protocol
)	O
is	O
not	O
available	O
in	O
DCCP	B-Protocol
.	O
</s>
<s>
A	O
DCCP	B-Protocol
connection	O
contains	O
acknowledgment	B-Protocol
traffic	O
as	O
well	O
as	O
data	O
traffic	O
.	O
</s>
<s>
Acknowledgments	B-Protocol
inform	O
a	O
sender	O
whether	O
its	O
packets	O
have	O
arrived	O
,	O
and	O
whether	O
they	O
were	O
marked	O
by	O
Explicit	O
Congestion	O
Notification	O
(	O
ECN	O
)	O
.	O
</s>
<s>
Acknowledgements	B-Protocol
are	O
transmitted	O
as	O
reliably	O
as	O
the	O
congestion	O
control	O
mechanism	O
in	O
use	O
requires	O
,	O
possibly	O
completely	O
reliably	O
.	O
</s>
<s>
DCCP	B-Protocol
has	O
the	O
option	O
for	O
very	O
long	O
(	O
48-bit	O
)	O
sequence	B-Protocol
numbers	I-Protocol
corresponding	O
to	O
a	O
packet	O
ID	O
,	O
rather	O
than	O
a	O
byte	O
ID	O
as	O
in	O
TCP	B-Protocol
.	O
</s>
<s>
The	O
long	O
length	O
of	O
the	O
sequence	B-Protocol
numbers	I-Protocol
aims	O
to	O
guard	O
against	O
"	O
some	O
blind	O
attacks	O
,	O
such	O
as	O
the	O
injection	O
of	O
DCCP-Resets	O
into	O
the	O
connection	O
"	O
.	O
</s>
<s>
DCCP	B-Protocol
is	O
useful	O
for	O
applications	O
with	O
timing	O
constraints	O
on	O
the	O
delivery	O
of	O
data	O
.	O
</s>
<s>
Such	O
applications	O
include	O
streaming	O
media	O
,	O
multiplayer	B-Application
online	I-Application
games	I-Application
and	O
Internet	B-Application
telephony	I-Application
.	O
</s>
<s>
such	O
applications	O
have	O
often	O
either	O
settled	O
for	O
TCP	B-Protocol
or	O
used	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
and	O
implemented	O
their	O
own	O
congestion-control	O
mechanisms	O
,	O
or	O
have	O
no	O
congestion	O
control	O
at	O
all	O
.	O
</s>
<s>
While	O
being	O
useful	O
for	O
these	O
applications	O
,	O
DCCP	B-Protocol
can	O
also	O
serve	O
as	O
a	O
general	O
congestion-control	O
mechanism	O
for	O
UDP-based	O
applications	O
,	O
by	O
adding	O
,	O
as	O
needed	O
,	O
mechanisms	O
for	O
reliable	O
or	O
in-order	O
delivery	O
on	O
top	O
of	O
UDP/DCCP	O
.	O
</s>
<s>
In	O
this	O
context	O
,	O
DCCP	B-Protocol
allows	O
the	O
use	O
of	O
different	O
,	O
but	O
generally	O
TCP-friendly	O
congestion-control	O
mechanisms	O
.	O
</s>
<s>
The	O
following	O
operating	O
systems	O
implement	O
DCCP	B-Protocol
:	O
</s>
<s>
The	O
DCCP	B-Protocol
generic	O
header	O
takes	O
different	O
forms	O
depending	O
on	O
the	O
value	O
of	O
X	O
,	O
the	O
Extended	O
Sequence	B-Protocol
Numbers	I-Protocol
bit	O
.	O
</s>
<s>
If	O
X	O
is	O
one	O
,	O
the	O
Sequence	B-Protocol
Number	I-Protocol
field	O
is	O
48	O
bits	O
long	O
,	O
and	O
the	O
generic	O
header	O
takes	O
16	O
bytes	O
,	O
as	O
follows	O
.	O
</s>
<s>
If	O
X	O
is	O
zero	O
,	O
only	O
the	O
low	O
24	O
bits	O
of	O
the	O
Sequence	B-Protocol
Number	I-Protocol
are	O
transmitted	O
,	O
and	O
the	O
generic	O
header	O
is	O
12	O
bytes	O
long	O
.	O
</s>
<s>
Data	O
Offset	O
(	O
8	O
bits	O
)	O
:	O
The	O
offset	O
from	O
the	O
start	O
of	O
the	O
packet	O
's	O
DCCP	B-Protocol
header	O
to	O
the	O
start	O
of	O
its	O
application	O
data	O
area	O
,	O
in	O
32-bit	O
words	O
.	O
</s>
<s>
Similarly	O
to	O
the	O
extension	O
of	O
TCP	B-Protocol
protocol	I-Protocol
by	O
multipath	O
capability	O
(	O
MPTCP	B-Protocol
)	O
also	O
for	O
DCCP	B-Protocol
the	O
multipath	O
feature	O
is	O
under	O
discussion	O
at	O
IETF	O
correspondingly	O
denoted	O
as	O
MP-DCCP	O
.	O
</s>
